Touchscreen panels (TSPs) are widely used in consumer electronic devices. Resistive and capacitive touchscreen panels have been widely adopted as input means for data processing devices, in particular for mobile devices such as smartphones, tablet computers and laptops. There is also growing interest in pressure sensing touchscreen panels, for example as described in US 2014/0008203 A1.
Regardless of the specific parameter(s) measured by a touchscreen panel to detect user inputs, three parameters which influence user experience are: power consumption, detection accuracy and responsivity. Power consumption influences the battery life-time of a mobile device including the touchscreen panel. Detection accuracy is important for efficient human-machine interactivity. High responsivity is significant for real-time applications. A factor related to all three parameters is the number of measured touch sensor areas. Touchscreen panels are commonly arranged to record user interactions or touch events using sensor areas arranged in a two-dimensional array or sensor areas provided by the intersections of a two-dimensional grid of electrodes. When fewer sensors are read, higher responsivity is obtained, and less power consumption is required. However, the resolution is reduced.
As used herein, the term “touch event” should cover a user interacting with a touchscreen by touching or pressing the touchscreen panel using a digit/finger or a suitable stylus. The signals output from a touchscreen panel having a two dimensional array or grid arrangement of sensor areas are collated into a corresponding array or frame. Such a frame may be presented as an image to help visualise the output signals from a touchscreen panel as a whole, with each pixel value of the frame corresponding to a signal level of a corresponding sensor area of the touchscreen panel.
For example, referring to FIG. 1, a full resolution frame Ffull may be acquired using the maximum resolution possible for a given touchscreen panel. In grayscale plots illustrating frames, regions labelled “LG” correspond to low signal levels, regions labelled “DG” correspond to intermediate signal levels and regions labelled “B” correspond to high signal level.
Alternatively, a down-sampled, or partial, frame Fdown may be acquired by sampling a subset of the available sensor areas, for example, a quarter (0.25) of the available sensor areas. Down-sampling the number of sensor areas may decrease power consumption and increase the speed of acquiring a touch signal frame, i.e. reduce the interval between acquiring successive frames. However, the resolution is decreased, which may result in reduced detection accuracy of touch events and/or errors in registering touch events.
In an ideal touchscreen panel, the signal frames Ffull, Fdown would register zero everywhere except at the locations of touch events. However, in practice, even when a two-dimensional array or grid of touch sensor areas is sampled in the absence of any touch event, almost all of the values acquired will be non-zero values corresponding to noise signals or offset values (e.g. DC or slowly varying). The value from each touch sensor area corresponds to a location or pixel value of the signal frame Ffull, Fdown. Consequently, signals resulting from touch events must be large enough to overcome such noise signals and offset values, which may require relatively larger excitation voltages to be used for accurate detection of touch events. High excitation voltages for acquiring a signal frame from a touchscreen panel contribute to increased power consumption.